(1) Field of the Invention
The present invention relates to a branch device for use in an optical multiplex system in an optical communication network, and more particularly to a branch device for use in an optical multiplex system for making more economical an optical communication network which incorporates such a branch device.
(2) Description of the Related Art
In an optical communication network, light outputted from one station can be branched by a branch device and transmitted to a plurality of stations. If such a branch device is incorporated in a submarine optical transmission system, then it is possible to branch signal light undersea and transmit it to a plurality of stations on land.
FIG. 10 of the accompanying drawings shows a conventional undersea branch device (Bu) 50. To the undersea branch device 50, there are connected cable ends A, B, and C of respective three transmission cables connected respectively to three stations. Specifically, upstream and downstream optical fibers (fiber pairs) are connected between the stations such that two optical fibers 51, 52 extend between the cable ends A and B, two optical fibers 53, 54 extend between the cable ends C and B, and two optical fibers 55, 56 extend between the cable ends B and A, all through the branch device 50. For transmitting signal light, each of the stations selects one of the connected optical fibers connected thereto depending on the direction in which the signal light is to be transmitted therefrom. Signal light transmitted between the three stations connected to the transmission cables can be branched by the branch device 50.
The conventional undersea branch device 50 requires many fiber cables to be installed because each combination of two stations needs a fiber pair of upstream and downstream optical fibers. Since one station is connected to two stations through respective fiber pairs, a repeater on each transmission cable needs to be of a dual-system configuration for amplifying signal light transmitted through such two fiber pairs. It is not possible to employ an inexpensive single-system repeater for amplifying signal light transmitted through a single fiber pair.